RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems etc. The signals amplified by the RF power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 megahertz (MHz) to 4 gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 300 MHz or higher.
RF power amplifiers are designed to provide linear operation without distortion. Input and output impedance matching circuits are used to match RF transistors that may have low input and output impedances (e.g., around 1 ohm or less for high power devices), to external transmission lines that provide RF signals to and from the RF transistor. These external transmission lines have characteristic impedances that are typically 50 ohms but could be any value as decided by a designer. The input and output matching circuits typically include inductive and capacitive elements that are used to provide impedance matching between the input and output of the RF power amplifier and the input and output of the RF transistor. The input and output matching circuits provide impedance matching for the signal frequencies that are amplified by the RF power amplifier, such as those in the 400 MHz to 4 GHz range.
The use of impedance matching circuits, however, can cause unintended consequences that occur outside of the range of signal frequencies that the impedance match is being provided for. For example, a typical output match network will include a blocking capacitor for blocking DC. The blocking capacitor in combination with the RF transistor drain bias inductance creates a low frequency resonance. This low frequency resonance causes the impedance in the low frequency region to increase. As a result, the frequency response of the RF power amplifier has a low frequency gain spike. Such a spike can appear anywhere from a few MHz to hundreds of MHz. The output of a nonlinear operation yields terms with frequencies at the sum and difference of the original signal frequencies, plus the original frequencies and multiples of the original frequencies, and those multiples are commonly referred to as harmonics. Current wireless signals have high modulation bandwidths. The second order distortion components of such wideband signals may fall in the region of the low frequency gain spike. Further, in most wireless communication applications distortion correction systems such as DPD or Digital Pre-Distortion are used. Such systems model the power amplifier, predict the non-linear performance and adjust the signal characteristics to reduce the distortion at the PA system output. The undesired high gain (or high impedance) in the baseband region due to the low frequency resonance negatively impacts the RF transistor and pre-distortion performance of the overall system.
A resonance in baseband frequency region causes a sharp change in gain at these low frequencies. The frequency at which the low frequency gain peak occurs is typically known as the video bandwidth of the RF power amplifier. Moreover, the magnitude of the gain peak also impacts the system performance. A higher magnitude of gain peak results in worse overall system performance. Additionally, the resonance in the baseband frequency region causes high peak voltages at the drain of RF transistors such as LDMOS (laterally-diffused metal-oxide semiconductor) transistors. These high peak voltages at the drain of the RF transistor can surpass the breakdown voltage of the device under certain conditions causing failures. Consequently, any increase of the gain peak within the low frequency baseband region can effectively reduce the ruggedness of the power device.